The effects of i.v. cytomegalovirus (CMV) immunoglobulin given for prophylaxis of CMV infections in recipients of allogeneic and autologous marrow transplants were evaluated in a randomized trial: 60 patients were randomly assigned to receive (30 patients) or not to receive (30 patients) CMV immunoglobulin for a period of 90 days after transplantation. As to the allografted patients, the cumulative incidence of asymptomatic and symptomatic CMV infections was significantly reduced in the CMV immunoglobulin-treated group as compared to the control group (56.5% versus 92.9%, P less than 0.05). No other statistically significant effect of CMV immunoglobulin could be found. In particular, the incidence of symptomatic CMV infections (including interstitial pneumonia), the mean delay of post-transplant viraemia and haematopoietic recovery were similar in the control and CMV immunoglobulin-treated groups. We conclude that prophylactic CMV immunoglobulin administration, as designed in our study, is no more than marginally effective and cannot be recommended without additional trials.